Add "Mutant NPCs" mod.

[SpencerMichaels]

This PR is split off from #22487. It depends on #22494.

This PR adds a Mutant NPCs mod which adds 21 new NPC classes: one generic mutant and 20 mutants targeted at each of the serum categories. These classes are similar to NPC_NONE, the generic wandering NPC, except that they have effectively drunk/injected several mutagens/serums before spawning. Only some mutations have an effect for NPCs right now, but there are enough to make, say, a bear mutant companion a much better fighter than a regular one.

Aside from adding these NPC classes, the mod doesn't do anything else yet — it's primarily intended to make use of some of my NPC mutation frameworks in the other related PRs (see #22494, #22495, and #22496). I also want to use the trait groups system to add non-mutation starting traits to the regular NPCs, but that will require lua support for groups, which I don't have yet. I hope to eventually add a mutant faction and some associated quests, although some of the ideas that I have in mind require some code additions as well as mods.

If anyone has recommendations for changes or additions, I'd be glad to hear them.

[SpencerMichaels]

Taking this off of WIP status because I think at least as a first version of the mod this is ready to merge. My only concern is that there might be too many mutants in random spawns because there are 21 new classes and the random selection is a uniform distribution… but since this is a mod, that might actually be what people want.

[kevingranade]

This is triggering the debugmsg at https://github.com/sxmichaels/Cataclysm-DDA/blob/ab428e1f3d05ae1047c44e963452ae67eafc3da7/src/npc_class.cpp#L253
Looks like the check wants the mutation category id, but the npc_class is specifying the category name, which is not the same. I'm seeing if I can apply a simple patch for this since I've let this sit for way too long...

[AMurkin]

@kevingranade Astyle regression: f5ef998#r27036628